1. Field of the Invention
This invention relates to a disc recording and/or reproducing apparatus employing a recording disc, such as a magneto-optical disc or an optical disc, as a recording medium. More particularly, it relates to a disc recording and/or reproducing apparatus on which a disc cartridge having the recording disc housed therein may be loaded as such, that is with the disc housed therein.
2. Description of the Related Art
There has hitherto been proposed a disc recording and/or reproducing apparatus on which a disc cartridge housing therein a recording disc such as a magneto-optical disc or an optical disc carrying pre-recorded information signals or information signals to be recorded may be loaded as such and in which the information signals may be recorded on the recording disc or the information signals pre-recorded on the disc may be reproduced from the disc.
As the recording medium for this type of the recording and/or reproducing apparatus, there are a play-only optical disc on which the information signals are pre-recorded and exclusively reproduced, and a magneto-optical disc in which the informations signals once recorded on the disc are erased to permit rerecording of new signals.
The play-only optical disc includes a disc substrate, fabricated by molding synthetic resin, such as transparent polycarbonate resin exhibiting light transmittancy, and a reflective layer of a metallic material, such as aluminum, deposited on one of the major surfaces of the disc substrate. On the major surface of the disc substrate, provided with the reflective layer of the disc substrate, a pattern of pits and lands corresponding to the informations signals is previously formed by, for example, injection molding, by means of a stamper. That is, the informations signals are recorded as the pattern of pits and lands formed by the stamper. The informations signals, recorded on the disc substrate in the form of the pattern of pits and lands, may be read out and reproduced by radiating a laser beam from the major surface of the optical disc opposite to the surface provided with the reflective layer, and by detecting the volume of reflected light by detecting means, such as photodetectors.
On the other hand, the magneto-optical disc enabling rerecording of information signals includes a disc substrate formed by molding a transparent synthetic resin, such as transparent polycarbonate resin, exhibiting light transmitting properties, similarly to the optical disc, and a signal recording layer, consisting in a magnetic layer, deposited on one of the major surfaces of the disc substrate. The information signals may be recorded on the magneto-optical disc by converging a laser beam emanating from an optical pickup device at a predetermined site of the signal recording layer and applying an external magnetic field on the irradiated site by an external magnetic field generating device. Meanwhile, the laser light beam is radiated on the signal recording layer through the disc substrate from the major surface opposite to the major surface provided with the signal recording layer. On the other hand, the external magnetic field is applied from the major surface of the magneto-optical disc opposite to the major surface thereof irradiated with the laser beam. The portion of the signal recording layer irradiated with the laser beam becomes devoid of coercivity by being heated to a temperature higher than the Curie temperature. The external magnetic field is applied to the disc in the state bereaved of coercivity so that the signal recording layer has its magnetic domain inverted in accordance with the external magnetic field. The information signals are recorded by allowing the disc to cool to fix the state of the magnetic domains.
In this manner, it becomes possible with the magneto-optical disc to record the information signals by modulating the direction of the external magnetic field or the intensity of the laser beam for inverting the magnetic domains of the signal recording layer in a controlled manner, or to erase or re-record information signals, while running the disc in rotation.
The disc recording and/or reproducing apparatus for recording information signals using the above-described magneto-optical disc is comprised of a rotating unit for rotating a magneto-optical disc in unison, a magnetic head for applying an external magnetic field to the magneto-optical disc, and an optical pickup unit for radiating a converged laser beam to the magneto-optical disc. The magnetic head and the optical pickup unit are arranged facing each other with the magneto-optical disc in-between. Besides, the magnetic head and the optical pickup unit are mounted, along with an arm member as coupling means, for movement radially of and across the inner and outer peripheries of the disc.
The magnetic head is comprised of a magnetic core and a coil wound about the magnetic core for generating a magnetic field. The magnetic head is mounted in close proximity to the signal recording layer with the interposition of e.g. a lubricious layer and a protective layer of a synthetic material. The optical pickup unit includes a light source, such as a laser diode, and an optical device, such as an object lens, for guiding and converging a light beam radiated from the light source. The optical pickup unit is arranged at a distance from the signal recording layer substantially equal to a focal length of an object lens converging the light beam so that the point of convergence of the light beam is at a boundary between the signal recording layer and the disc substrate.
With such disc recording and/of reproducing apparatus, employing a magneto-optical disc as a recording medium, it has been customary that not only the information signals can be recorded on the disc, but also the information signals can be reproduced from the disc. The information signals may be reproduced from the magneto-optical disc by radiating a converged light beam in a linearly polarized state and detecting the direction of polarization of the light beam reflected from the signal recording layer. That is, the light beam when reflected by the surface of the signal recording layer has its direction of polarization changed under the Kerr effect. Consequently, the change in the direction of magnetization in the signal recording layer can be read by detecting the direction of polarization of the reflected light beam.
Detection of the reflected light beam is by a detector and a photodetector arranged within the optical pickup unit. The detector is a so-called Wollaston prism of a 1/2 wavelength plate etc. for converting the difference in the direction of light polarization into changes in light intensity. The photodetector is a photodiode of a phototransistor for converting the light intensity of the received light beam into corresponding electrical signals.
The disc recording and/or reproducing apparatus, employing the magneto-optical disc as a recording medium, enables the information signals recorded on a reproduce-only optical disc as a pattern of pits and lands to be reproduced. That is, the information signals recorded in the form of the pattern of pits and lands are reproduced by detecting the volume of light reflected from the optical disc.
When recording the information signals on the magneto-optical disc in the above-mentioned recording and/or reproducing apparatus, both the magnetic head and the optical pickup unit are run in operation. However, when reproducing the information signals from the magneto-optical disc or from a reproduce-only optical disc with the disc recording and/or reproducing apparatus, only the optical pickup unit is driven in operation, while the magnetic head is not driven in operation.
The sliding contact of the magneto-optical disc with the magnetic head causes wear to the magnetic head and to the magneto-optical head. Consequently, if the magnetic head is slidingly contacted with the magneto-optical disc a large number of times, the magnetic head or the magneto-optical disc is lowered in durability.
For this reason, the code information indicative of the type of the recording disc previously recorded in a table-of-contents (TOC) area provided in the radially inner most zone in the recording region of the disc is reproduced on loading the disc cartridge for finding if the disc loaded in the disc recording and/or reproducing apparatus is a magneto-optical disc, that is a re-recordable disc, or an optical disc, that is a read-only optical disc.
If the code information indicates that the disc loaded is the magneto-optical disc, the magnetic head is driven in a direction of approaching the magnetic head towards the magneto-optical disc, whereas, if the code information indicates that the disc loaded is the reproduce-only optical disc, the magnetic head is not driven in operation and is positioned at a distance from the disc.
However, the disc recording and/or reproducing apparatus for discerning the disc type by the code information recorded on the recording disc for driving the magnetic head towards and away from the recording disc, it is necessary to provide special circuitry for discerning the disc type or a special mechanism for driving the magnetic head, thus complicating the structure. Besides, the recording or reproducing operation can not be performed quickly because of time involved in discerning the disc type on loading the disc cartridge.